Fix `SetMemoryRelation#equals` and properly test it.
Refactor Predicate-based restrict support a little bit.

This commit moves Predicate to `jalf`, as it is part of the public
API. Predicate implementation (i.e. `Eq`) moves to a private
subpackage. Also, Eq now supports various scenarios in an unambiguous
way, by properly distinguishing between values (java Objects) and
attribute names (AttrName specifically).